Revert "Reland "Remove tracking of which animating elements exist.""

This reverts commit 85c7ec9d9caf533428c6b40f85d3b03de4c4f81c.

Reason for revert: Causing crashes due to animating non-existent property nodes https://crbug.com/1305503 https://crbug.com/1305487

Original change's description:
> Reland "Remove tracking of which animating elements exist."
>
> The crashes seen last time should have been resolved by https://chromium-review.googlesource.com/c/chromium/src/+/3504027, https://chromium-review.googlesource.com/c/chromium/src/+/3498651, and https://chromium-review.googlesource.com/c/chromium/src/+/3517088
>
> Original change's description:
> > Remove tracking of which animating elements exist.
> >
> > Prior to property trees, composited animations were created before their
> > corresponding layers existed. However, the blink lifecycle now ensures
> > that composited animations are created in sync with their property tree
> > nodes such that they always exist in the tree. Removing this code saves
> > time and will unlock further simplifications of the code.
> >
> > The ui animation infrastructure eagerly creates layers before their
> > animations so it doesn't have this issue and the LayerAnimator class
> > ensures that animations are cleaned up on Layer removal.
> >
> > Bug: 900241
> > Change-Id: If854cf67ba8bd21e727e42475d26ce1a846a7674
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3200347
> > Reviewed-by: David Baron <dbaron@chromium.org>
> > Reviewed-by: Philip Rogers <pdr@chromium.org>
> > Commit-Queue: Robert Flack <flackr@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#975833}
>
> Bug: 900241
> Change-Id: I59b620322c77bb98d935f8785fbb87afad8e7be1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3498292
> Reviewed-by: David Baron <dbaron@chromium.org>
> Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
> Commit-Queue: Robert Flack <flackr@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#980063}

Bug: 900241
Change-Id: I73c960f092773d193639c64322881b655ee00505
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3517538
Auto-Submit: Robert Flack <flackr@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#980670}

• cc/animation/animation_host.cc[diff]
• cc/animation/animation_host.h[diff]
• cc/animation/animation_timeline.cc[diff]
• cc/animation/element_animations.cc[diff]
• cc/animation/element_animations.h[diff]
• cc/animation/element_animations_unittest.cc[diff]
• cc/animation/keyframe_effect.cc[diff]
• cc/animation/keyframe_effect.h[diff]
• cc/animation/scroll_offset_animations_impl.cc[diff]
• cc/animation/scroll_offset_animations_impl.h[diff]
• cc/layers/layer_impl.cc[diff]
• cc/test/animation_timelines_test_common.cc[diff]
• cc/test/mock_mutator_host.h[diff]
• cc/trees/layer_tree_host.cc[diff]
• cc/trees/layer_tree_host.h[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_host_impl_unittest.cc[diff]
• cc/trees/layer_tree_host_unittest_animation.cc[diff]
• cc/trees/layer_tree_impl.cc[diff]
• cc/trees/layer_tree_impl.h[diff]
• cc/trees/mutator_host.h[diff]
• cc/trees/mutator_host_client.h[diff]
• third_party/blink/renderer/core/animation/animation.cc[diff]
• third_party/blink/renderer/core/animation/compositor_animations.cc[diff]
• third_party/blink/renderer/platform/graphics/compositing/paint_artifact_compositor.cc[diff]
• third_party/blink/web_tests/TestExpectations[diff]